The invention relates generally to systems for handling semiconductor materials such as semiconductor wafers during processing in furnace tubes or the like and more particularly to an improved system for loading and unloading semiconductor materials from a furnace tube used in semiconductor processing.
The processing of silicon and other semiconductor materials to form electronic components involves numerous fabrication steps, some of which require the heating of the semiconductor material to high temperatures. Semiconductor material is generally processed in the form of wafers, which are flat, usually circular discs. Each disc may contain hundreds or thousands of tiny semiconductor components. Cleanliness is of the utmost importance in processing semiconductor wafers because due to the small size of the components, a single speck of dust or even a molecule of contaminant might destroy a component on the wafer. The wafers are therefor processed in ultra clean environments from which potential contaminants are eliminated.
The processing of semiconductor wafers usually includes several heating operations for which the wafers must be loaded into special hightemperature furnaces. The temperatures required, sometimes ranging above 1,000.degree. C., necessitate the use of special materials which neither melt nor release contaminants when heated. One preferred material for use in handling semiconductor wafers is quartz. Those components which must remain in the furnace with the wafers during processing, including the racks or "boats" on which the wafers are retained, are formed of quartz. Other materials suitable for use in semiconductor furnaces include polysilicon and silicon carbide. Together with quartz, these materials, and others possessing similar properties suitable for use in semiconductor processing furnaces, are referred to herein as semiconductor grade materials.
Semiconductor processing furnaces generally consist of one or more elongated cylindrical chambers arranged horizontally with an opening at one end through which loading and unloading of the wafers takes place. The chambers, which are generally formed of quartz, are referred to herein as furnace tubes. Heating elements are positioned exterior of the furnace tubes, and for some processes, gas discharge jets are arranged along the interior of the tubes for the injection of gases. Prior art loading systems for inserting semiconductor wafers into a furnace tube generally consist of wheeled racks made of quartz on which boats and wafers are supported. The racks are rolled into the furnace tube and remain inside during the heating process. Such wheeled racks tend to release particles of quartz into the atmosphere of the furnace tube due to friction. Accordingly, less contaminating techniques have been developed.
One improved technique for loading furnace tubes is to support the wafers in a cantilevered manner within the tube during heating, thus eliminating contact between materials within the tube which could raise contaminating particles. In general this is accomplished by inserting an elongated carrier into the furnace tube carrying the wafers. Because of the high temperature environment which must be maintained for long periods, it has been found that the material from which the carrier is formed will often sag if carrying too great a weight. One way of avoiding this problem is to deposit the carrier on the floor of the furnace tube, where it remains during the heating process. Setting the carrier or another object on to the floor of the furnace tube does pose some risk of raising contaminants into the atmosphere, but the problem is far less than in loading systems which involve friction between parts.
For certain processes, such as those requiring rapid temperature response, the presence of a relatively massive supporting carrier in the furnace tube, either suspended or resting on the floor, presents disadvantages. Other systems for supporting semiconductor wafers in a furnace tube encounter different problems. Systems which support wafers throughout the heating cycle must generally use reinforcing material in the supporting structure, since quartz will sag under heavy loading at high temperatures. Problems associated with the use of different materials in processes involving large temperature gradients must then be dealt with. Foreign materials, even when sealed in quartz, present a potential for contamination due to molecular migration through the quartz. Reinforcing with metals requires special handling of the equipment, since most metals cannot withstand the temperatures in the furnace tube. Loading must therefore be accomplished quickly enough to prevent sag or, alternatively, the furnace must be cooled sufficiently to permit proper loading. Those alternatives present problems for the wafers which include a potential for undesirable thermal shock, increased vibration and increased downtime as the furnace is cooled and reheated.
It would be advantageous to provide a furnace loading system which provides for non-contact loading with improved performance. In particular, it would be advantageous to have a loading system which could withstand high temperatures within the furnace during loading and unloading and which would leave behind less additional mass to be heated by the furnace. Such a system would then require less heating and cooling of the furnace and offer greater control and uniformity in the processing of the semiconductor wafers.